The present invention relates to a brake pressure control apparatus for a hydraulic automotive vehicle brake system and includes (1) a brake pressure master unit, (2) at least one brake pressure slave unit connected to the brake pressure master unit through a pressure line for the actuation of a brake, (3) a pressure-controlled inlet valve which is positioned in the pressure line and through which the brake pressure master unit is connected, in a first switching position of the inlet valve, unrestrictedly and, in a second switching position of the inlet valve, through a restrictor orifice to the brake pressure slave unit. The pressure-controlled inlet valve has a control piston which, in one direction, is subject to the pressure in the pressure line and, in the opposite direction, is subject to the action of a spring and of the pressure in a return line which is connected to the valve and which contains an outlet valve which is controllable by a control device, which blocks the return line in its position of rest and which releases the return line in its switching position.
Brake pressure control apparatus of this type serve in the automatic control of the brake pressure depending on the rotary motion of one or a plurality of wheels of a vehicle. The rotary motion of the wheels is monitored by the control apparatus to avoid the blocking of the wheels by the braking operation or by a racing of the driven wheels when starting.
A brake pressure control apparatus of this type is known from U.S. Pat. No. 4,715,666. In this brake pressure control apparatus, the inlet valve is arranged as a flow control valve, the restrictor orifice being positioned in a longitudinal bore of the control piston and the control piston being subject exclusively to the pressure of the pressure master unit and to the pressure in the return line. The state-of-the-art flow control valve has the disadvantage that the effect of a pressure reduction in the pressure line leading to the brake pressure slave unit upon the aperture of the outlet valve takes place only with a delay, because the control piston must first be shifted before hydraulic pressure fluid can be withdrawn from the brake pressure slave unit. In view of the short control times which are required for pressure reductions and for pressure increases in slip control action of the brake, such a delay of the pressure reduction adversely affects the control function.
Furthermore, a precise positioning of the connecting bores interacting with the control piston is necessary in the prior-art flow control valve which boosts the costs of manufacture.
Another hydraulic brake system for automotive vehicles with anti-locking control is known from the German patent application published without examination, U.S. Pat. No. 3,919,842 in which the brake pressure control apparatus has a throttle valve in the line leading from the brake pressure master unit to the brake pressure slave unit. Depending on the pressure at the outlet of a pump circuit which starts to conduct on the outset of a control action, the throttle valve is switched from a first switching position with free passage into a second switching position in which the connection between the brake pressure slave unit and the brake pressure master unit is restricted. In such an arrangement, the outlet valve may be connected directly to the brake pressure slave unit, so that a rapid pressure reduction is possible. However, additional non-return valves are required to prevent any pressure reduction in the control line of the throttle valve upon actuation of the brake without brake pressure control action.